This method is used by many non-Intel processors. Certain regions of memory is reserved. These regions of memory do not map to memory chips, but I/O chips.
The region of memory mapped to I/O can be hardwared (a chip select line is connected from the MMU to an I/O chip) or programmed in case of a programmable MMU.
This method is characterized by only instructions that access memory locations. This characteristic, in return, makes it possible to perform I/O operations using general pointer dereferencing (once the pointers are initialized or declared const).